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Abstract:

A pneumatic tire comprises a tread portion divided into a crown land
region, two middle land regions and two shoulder land regions. The crown
land region and middle land regions are each divided into triangular
blocks by axial grooves arranged in a zigzag fashion. The shoulder land
regions are each divided into shoulder blocks by axial grooves extending
at an angle of not less than 70 degrees. The shoulder block is subdivided
into an axially inner part and an axially outer part by a secondary
groove extending at an angle of not more than 10 degrees. The outer part
is provided with sipes extending at an angle of not less than 70 degrees,
each angle with respect the circumferential direction.

Claims:

1. A pneumatic tire comprising a tread portion provided on each side of
the tire equator with an axially inner crown main groove and an axially
outer shoulder main groove which extend continuously in the tire
circumferential direction so that the tread portion is divided into a
crown land region between the crown main grooves, a pair of middle land
regions between the crown main grooves and the shoulder main grooves, and
a pair of shoulder land regions between the shoulder main grooves and
tread edges, wherein the crown land region is provided with crown axial
grooves which are narrow grooves and/or sipes extending across the entire
width of the crown land region and arranged in a zigzag fashion so as to
divide the crown land region into a plurality of crown blocks having
triangular configurations, the middle land regions are each provided with
middle axial grooves which are narrow grooves and/or sipes extending
across the entire width of the middle land region and arranged in a
zigzag fashion so as to divide the middle land region into a plurality of
middle blocks having triangular configurations, the shoulder land regions
are each provided with shoulder axial grooves extending across the entire
width of the shoulder land region at an angle of not less than 70 degrees
with respect to the tire circumferential direction so as to divide the
shoulder land region into a plurality of shoulder blocks, the shoulder
blocks are each provided with a shoulder secondary groove extending at an
angle of not more than 10 degrees with respect to the tire
circumferential direction and disposed at a distance of 3 to 15 mm
axially outward from the shoulder main groove so as to subdivide the
shoulder block into an axially inner part and an axially outer part, and
the outer part is provided with shoulder sipes extending at an angle of
not less than 70 degrees with respect to the circumferential direction.

2. The pneumatic tire according to claim 1, wherein the depth of the
shoulder secondary groove is 0.5 to 7.0 mm, and the bottom of the
shoulder secondary groove is provided with a groove-bottom sipe extending
along the shoulder secondary groove.

3. The pneumatic tire according to claim 2, wherein the groove-bottom
sipe has closed ends positioned at a distance of from 1.0 to 8.0 mm from
respective circumferential ends of the shoulder secondary groove.

4. The pneumatic tire according to claim 1, wherein the width of the
shoulder secondary groove is more than its depth.

5. The pneumatic tire according to claim 1, wherein the shoulder axial
groove is provided with a tie bar rising from the groove bottom at a
position on its shoulder main groove side.

6. The pneumatic tire according to claim 1, wherein the crown axial
grooves are sipes having a width of from 0.3 to 1.6 mm.

7. The pneumatic tire according to claim 1, wherein the crown blocks are
each provided with a crown slot extending from the crown main groove
toward the tire equator and terminating within the crown block.

8. The pneumatic tire according to claim 1, wherein the middle axial
grooves are first middle sipes having a width of from 0.3 to 1 6 mm and
middle narrow grooves having a width more than 1.6 mm and not more than
7.0 mm, and the first middle sipes and the middle narrow grooves are
arranged alternately in the tire circumferential direction.

9. The pneumatic tire according to claim 1, wherein the middle blocks are
each provided with a second middle sipe extending from the crown main
groove or the shoulder main groove and terminating within the middle
block.

10. The pneumatic tire according to claim 2, wherein the width of the
shoulder secondary groove is more than its depth.

11. The pneumatic tire according to claim 3, wherein the width of the
shoulder secondary groove is more than its depth.

12. The pneumatic tire according to claim 2, wherein the shoulder axial
groove is provided with a tie bar rising from the groove bottom at a
position on its shoulder main groove side.

13. The pneumatic tire according to claim 3, wherein the shoulder axial
groove is provided with a tie bar rising from the groove bottom at a
position on its shoulder main groove side.

14. The pneumatic tire according to claim 4, wherein the shoulder axial
groove is provided with a tie bar rising from the groove bottom at a
position on its shoulder main groove side.

15. The pneumatic tire according to claim 2, wherein the crown axial
grooves are sipes having a width of from 0.3 to 1.6 mm.

16. The pneumatic tire according to claim 3, wherein the crown axial
grooves are sipes having a width of from 0.3 to 1.6 mm.

17. The pneumatic tire according to claim 4, wherein the crown axial
grooves are sipes having a width of from 0.3 to 1.6 mm.

18. The pneumatic tire according to claim 5, wherein the crown axial
grooves are sipes having a width of from 0.3 to 1.6 mm.

19. The pneumatic tire according to claim 2, wherein the crown blocks are
each provided with a crown slot extending from the crown main groove
toward the tire equator and terminating within the crown block.

20. The pneumatic tire according to claim 3, wherein the crown blocks are
each provided with a crown slot extending from the crown main groove
toward the tire equator and terminating within the crown block.

Description:

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a pneumatic tire, more
particularly to a tread pattern suitable for passenger car tires capable
of improving on-the-snow performance, without sacrificing steering
stability on dry pavement.

[0002] In order to improve on-the-snow performance without sacrificing
steering stability on dry pavement, a pneumatic tire (a) having a tread
pattern shown in FIG. 6 has been proposed in Japanese Patent Application
Publication JP-H07-186626.

[0003] The tread portion (b) of this pneumatic tire (a) is provided with a
pair of crown main grooves (d) and a pair of shoulder main groove (e) so
as to divide the tread portion (b) into a crown land region (f), a pair
of middle land regions (g), and a pair of shoulder land regions (h).

[0004] The crown land region (f), middle land regions (g) and shoulder
land regions (h) are divided into a plurality of blocks (f1, g1 and h1)
by narrow grooves (j) and sipes (k1) which are inclined with respect to
the circumferential direction.

[0005] Each of the blocks (f1-h1) is provided with a sipe (k2) inclined
with respect to the tire circumferential direction.

[0006] Therefore, by the edges of the sipes (k1 and k2), on-the-snow
performance of the pneumatic tire (a) can be improved, but the decrease
in the rigidity of the land regions (f-h) is little since the width of
the sipes is very narrow, therefore, the steering stability on dry
pavement is not sacrificed.

[0007] In this tread pattern, however, wandering and one-side drifting of
the vehicle is liable to occur when running on snowy roads covered with
compacted snow.

SUMMARY OF THE INVENTION

[0008] It is therefore, an object of the present invention to provide a
pneumatic tire, in which on-the-snow performance including anti-wandering
and one-side drifting performance can be improved, without sacrificing
the steering stability on dry pavement.

[0009] According to the present invention, a pneumatic tire comprises a
tread portion provided on each side of the tire equator with an axially
inner crown main groove and an axially outer shoulder main groove which
extend continuously in the tire circumferential direction so as to divide
the tread portion into a crown land region between the crown main
grooves, a pair of middle land regions between the crown main grooves and
the shoulder main grooves, and a pair of shoulder land regions between
the shoulder main grooves and tread edges, wherein

[0010] the crown land region is provided with crown axial grooves which
are narrow grooves and/or sipes extending across the entire width of the
crown land region and arranged in a zigzag fashion so as to divide the
crown land region into a plurality of crown blocks having triangular
configurations,

[0011] the middle land regions are each provided with middle axial grooves
which are narrow grooves and/or sipes extending across the entire width
of the middle land region and arranged in a zigzag fashion so as to
divide the middle land region into a plurality of middle blocks having
triangular configurations,

[0012] the shoulder land regions are each provided with shoulder axial
grooves extending across the entire width of the shoulder land region at
an angle of not less than 70 degrees with respect to the tire
circumferential direction so as to divide the shoulder land region into a
plurality of shoulder blocks,

[0013] the shoulder blocks are each provided with a shoulder secondary
groove extending at an angle of not more than 10 degrees with respect to
the tire circumferential direction and disposed at a distance of 3 to 15
mm axially outward from the shoulder main groove so as to subdivide the
shoulder block into an axially inner part and an axially outer part, and

[0014] the outer part is provided with shoulder sipes extending at an
angle of not less than 70 degrees with respect to the circumferential
direction.

[0015] Therefore, in the pneumatic tire according to the present
invention, as the crown axial grooves and middle axial grooves are
arranged in the zigzag fashions, the circumferential component of the
groove edges is increased in the tread central region, and thereby
on-the-snow performance can be improved. Further, the crown axial grooves
and middle axial grooves are relatively narrow in groove width,
therefore, the decrease in the rigidity of the crown land region and
middle land regions due to the provision of the axial grooves can be
minimized, and the deterioration in the steering stability on dry
pavement can be prevented.

[0016] Since the shoulder secondary grooves are positioned on the tire
equator side of the shoulder land regions where, during cornering, the
ground pressure becomes relatively high and the ground contacting length
becomes relatively long, the circumferential component of the edges of
the shoulder secondary grooves functions effectively, and the cornering
performance on snowy roads can be improved.

[0017] Further, by the shoulder sipes disposed in the axially outer parts
of the shoulder blocks, the traction on snowy roads can be greatly
increased.

[0018] Furthermore, as the angle of the shoulder secondary grooves is set
to be at most 10 degrees, the shoulder secondary grooves are
substantially parallel with the tire circumferential direction. As a
result, wandering and one-side drifting of the vehicle during running on
snowy roads can be effectively suppressed.

[0019] In this application including specification and claims, various
dimensions, positions and the like of the tire refer to those under a
normally inflated unloaded condition of the tire unless otherwise noted.

[0020] The normally inflated unloaded condition is such that the tire is
mounted on a standard wheel rim and inflate to a standard pressure but
loaded with no tire load.

[0021] The undermentioned normally inflated loaded condition is such that
the tire is mounted on the standard wheel rim and inflate to the standard
pressure and loaded with the standard tire load.

[0022] The standard wheel rim is a wheel rim officially approved or
recommended for the tire by standards organizations, i.e. JATMA (Japan
and Asia), T&RA (North America), ETRTO (Europe), TRAA (Australia),STRO
(Scandinavia), ALAPA (Latin America), ITTAC (India) and the like which
are effective in the area where the tire is manufactured, sold or used.

[0023] The standard pressure and the standard tire load are the maximum
air pressure and the maximum tire load for the tire specified by the same
organization in the Air-pressure/Maximum-load Table or similar list. For
example, the standard wheel rim is the "standard rim" specified in JATMA,
the "Measuring Rim" in ETRTO, the "Design Rim" in TRA or the like. The
standard pressure is the "maximum air pressure" in JATMA, the "Inflation
Pressure" in ETRTO, the maximum pressure given in the "Tire Load Limits
at various cold Inflation Pressures" table in TRA or the like. The
standard load is the "maximum load capacity" in JATMA, the "Load
Capacity" in ETRTO, the maximum value given in the above-mentioned table
in TRA or the like. In case of passenger car tires, however, the standard
pressure and standard tire load are uniformly defined by 180 kPa and 88%
of the maximum tire load, respectively.

[0024] The term "tread width TW" is the axial distance between the tread
edges Te measured in the normally inflated unloaded condition of the
tire. The tread edges Te are the axial outermost edges of the ground
contacting patch (camber angle=0) in the normally inflated loaded
condition.

[0025] The term "sipe" means a groove having a very narrow width which is
usually 0.3 to 1.5 mm or a cut having substantially no groove width
unless otherwise noted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a developed view of a part of the tread portion of a
pneumatic tire according to an embodiment of the present invention.

[0027]FIG. 2 is a cross sectional view of a tread shoulder portion taken
along line A-A of FIG. 1.

[0028] FIG. 3(a) is an enlarged view of a part of the shoulder block row
shown in FIG. 1.

[0029] FIG. 3(b) is a cross sectional view taken along line B-B in FIG.
3(a).

[0030]FIG. 4 is an enlarged view of a part of the crown block row shown
in FIG. 1.

[0031]FIG. 5 is an enlarged view of a part of the middle block row shown
in FIG. 1.

[0032]FIG. 6 is a developed view of a part of the tread portion of a tire
used in the undermentioned comparative test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Embodiments of the present invention will now be described in
detail in conjunction with the accompanying drawings.

[0034] The pneumatic tire according to the present invention comprises a
tread portion 2, a pair of sidewall portions, a pair of bead portions, a
carcass extending between the bead portions, a tread reinforcing belt
disposed radially outside the carcass in the tread portion as usual.

[0035] The present invention is suitably applied to passenger car tires
and the like. FIG. 1 shows the tread portion 2 of such a passenger car
tire as an embodiment of the present invention. In the tread portion 2 in
this embodiment, by the undermentioned grooves and sipes, there is formed
a bidirectional tread pattern which is symmetry with respect to a point
on the tire equator c.

[0036] The tread portion 2 is provided on each side of the tire equator c
with an axially inner crown main grooves 3 and an axially outer shoulder
main grooves 4 which extend continuously in the tire circumferential
direction so that the tread portion 2 is axially divided into a crown
land region 5 between the crown main grooves 3, a pair of middle land
regions 6 between the crown main grooves 3 and the shoulder main grooves
4, and a pair of shoulder land regions 7 between the shoulder main
grooves 4 and tread edges Te.

[0037] Each main groove 3, 4 may be formed in a various configuration such
as wavy configuration and zigzag configuration. In this embodiment,
however, the crown main grooves 3 and shoulder main grooves 4 are each
formed in a straight configuration. The use of such straight grooves 3
and 4 is desirable because vehicle's unstable motions such as wandering
when applying brake, drift to one side and the like can be suppressed to
provide steering stability.

[0038] The width w1 of the crown main grooves 3 and the width w2 of the
shoulder main grooves 4 are preferably set in a range of from 3 to 10% of
the tread width Tw.

[0039] The depth D1 of the crown main grooves 3 and the depth D2 of the
shoulder main grooves 4 are preferably set in a range of from 6 to 10 mm.

[0040] If the width w1 and w2 and/or depth D1 and D2 exceed the respective
upper limits, there is a possibility that the rigidity of the land
regions 5, 6 and 7 becomes insufficient. If exceed the respective lower
limits, it becomes difficult for the snow packed into the grooves to be
self discharged.

[0041] The axial distance L1 of the center line G1 of the crown main
groove 3 from the tire equator c is preferably set in a range of not less
than 3%, more preferably not less than 6%, but not more than 20%, more
preferably not more than 16% of the tread width Tw.

[0042] The axial distance L2 of the center line G2 of the shoulder main
groove 4 from the tire equator C is preferably set in a range of not less
than 15%, more preferably not less than 20%, but not more than 40%, more
preferably not more than 35% of the tread width Tw.

[0043] Therefore, the rigidity balance between the land regions 5-7 is
improved to improve the steering stability.

[0044] The above-mentioned crown land region 5 is provided with crown
axial grooves 8. The crown axial grooves 8 are narrow grooves and/or
sipes extending across the entire width of the crown land region 5 and
arranged in a zigzag formation so that the crown land region 5 is divided
into a plurality of triangular crown blocks 5a.

[0045] The middle land region 6 is provided with middle axial grooves 9.
The middle axial grooves 9 are narrow grooves and/or sipes extending
across the entire width of the middle land region 6 and arranged in a
zigzag formation so that the middle land region 6 is divided into a
plurality of triangular middle blocks 6a.

[0046] The shoulder land region 7 is provided with shoulder axial grooves
10. The shoulder axial grooves 10 extend across the entire width of the
shoulder land region 7 so that the shoulder land region 7 is divided into
a plurality of shoulder blocks 7a.

[0047] In the crown land region 5 and middle land regions 6, therefore, by
the crown axial grooves 8 and middle axial grooves 9 arranged in zigzag
formations, the circumferential component of the groove edges is greatly
increased, and on-the-snow performance can be improved.

[0048] Further, as the crown land region 5 and middle land regions 6 are
divided into the triangular crown blocks 5a and triangular middle blocks
6a, the rigidity of the blocks 5a and 6a as a whole in the
circumferential direction and axial direction is evened, and the rigidity
balance between the crown land region 5 and middle land regions 6 is
improved. As the crown axial grooves 8 and middle axial grooves 9 are
relatively narrow in groove width, the decrease in the rigidity of each
land region 5, 6 can be minimized. Accordingly, the steering stability on
dry pavement can be effectively maintained.

[0049] As shown in FIG. 3(a), the angle θ1 of the shoulder axial
groove 10 with respect to the tire circumferential direction is set to be
at least 70 degrees, preferably at least than 75 degrees. If the angle
θ1 is less than 70 degrees, the lateral stiffness (rigidity) of the
shoulder blocks 7a becomes insufficient for the shoulder 1 and region 7
which is subjected to large ground pressure during cornering, and the
shearing force of the snow packed into the shoulder axial grooves 10
decreases.

[0050] The angle θ1 may be constant along the entire length of the
shoulder axial groove 10, but preferably the angle θ1 is gradually
increased toward the tread edge Te. Thereby, the lateral stiffness is
further increased near the tread edge Te which is subjected to a more
large ground pressure during cornering, and the steering stability can be
further improved. More preferably, the angle θ1 is set to be not
more than 88 degrees in view of the balance between the lateral stiffness
(rigidity) of the shoulder block 7a and the cornering performance.

[0051] In order to derive the rigidity of the shoulder block 7a and the
shearing force of the packed snow in a well balanced manner, the width w3
of the shoulder axial groove 10 is preferably set in a range of not less
than 1.0%, more preferably not less than 2.0%, but not more than 8.0%,
more preferably not more than 5.0% of the tread width Tw. Further, the
depth D3 of the shoulder axial groove 10 is preferably set in a range of
not less than 40%, more preferably not less than 50%, but not more than
100%, more preferably not more than 90% of the depth D2 of the shoulder
main grooves 4.

[0052] Each of the shoulder blocks 7a is provided with a single shoulder
secondary groove 13 in order to axially subdivide the shoulder block 7a
into an axially inner part 11 and an axially outer part 12.

[0053] The angle θ2 of the shoulder secondary groove 13 with respect
to the tire circumferential direction has to be at most 10 degrees,
preferably not more than 5 degrees, more preferably 0 degree. If the
angle θ2 exceed 10 degrees, the circumferential component of the
groove edges decreases, and the cornering performance on snowy roads is
deteriorated, and it becomes hard to control the wandering and one-side
drifting.

[0054] The axial distance L3 between the shoulder secondary groove 13 and
the shoulder main groove 4 (namely, the axial width of the inner part 11)
has to be in a range of not less than 3 mm, preferably not less than 4
mm, more preferably not less than 5 mm, but, not more than 15 mm,
preferably not more than 13 mm, more preferably not more than 11 mm.

[0055] If the distance L3 is less than 3 mm, the lateral stiffness
(rigidity) of the inner part 11 becomes insufficient, and wear tends to
concentrate in the inner parts 11. If the distance L3 is more than 15 mm,
the lateral stiffness (rigidity) of the outer parts 12 subjected to large
ground pressure during cornering is decreased, and the steering stability
is deteriorated.

[0056] As shown in FIG. 2, the depth D4 of the shoulder secondary groove
13 is preferably set in a range of not less than 0.5 mm, more preferably
not less than 1.0 mm, but not more than 7.0 mm, more preferably not more
than 6.0 mm. Therefore, the rigidity of the shoulder block 7a can be
maintained, while achieving self-discharging of snow packed into the
shoulder secondary grooves 13, and on-the-snow performance and steering
stability on dry pavement can be secured in a well balanced manner.

[0057] In order to effectively derive the above-mentioned effects, the
depth D4 of the shoulder secondary groove 13 is preferably less than the
width w4 of the shoulder secondary groove 13. More specifically, the
ratio D4/w4 of the depth D4 to the width w4 is preferably set in a range
of not less than 40%, more preferably not less than 50%, but not more
than 98%, more preferably not more than 90%.

[0058] Preferably, the outer part 12 of each of the shoulder blocks 7a is
provided with shoulder sipes 15 extending from the axial outside of the
tread edge Te toward the shoulder secondary groove 13 but terminating
without reaching thereto.

[0059] In this embodiment, two shoulder sipes 15 are formed so as to
divide the circumferential dimension of the outer part 12 into three
equi-parts.

[0060] The shoulder sipes 15 have to extend at an angle θ3 of not
less than 70 degrees, preferably not less than 75 degrees with respect to
the circumferential direction.

[0061] Such shoulder sipes 15 can increase the traction performance during
straight running, without excessively decreasing the block rigidity of
the outer part 12, therefore, on-the-snow performance can be improved. If
the angle θ3 is less than 70 degrees, the effect to improve the
traction on snowy roads decreases.

[0062] In order to secure the cornering performance on snowy roads and the
traction performance during straight running in a well balanced manner,
the angle θ3 is preferably set be not more than 88 degrees.

[0063] In this embodiment, the angle θ3 of the shoulder sipes 15 is
gradually increased toward the tread edge Te. Therefore, the shoulder
sipes 15 gradually increase the lateral stiffness (rigidity) toward the
tread edge Te, and the cornering performance on snowy roads can be
improved.

[0064] If the axial length L4 of the shoulder sipes 15 is too long, the
rigidity of the outer part 12 is decreased, and there is a possibility
that the steering stability on dry pavement is deteriorated. If too
short, the edge length becomes insufficient.

[0065] The axial length L4 of the shoulder sipe 15 is preferably not less
than 50%. more preferably not less than 60%, but not more than 98%, more
preferably not more than 95% of the axial width ws1 of the outer part 12.

[0066] The groove width w6 of the shoulder sipe 15 is preferably set in a
range of not less than 0.2 mm, more preferably not less than 0.3 mm, but
not more than 1.5 mm, more preferably not more than 1.2 mm.

[0067] It is preferable that the bottom 13s of the shoulder secondary
groove 13 is provided with a groove-bottom sipe 16 extending along the
shoulder secondary groove 13.

[0068] The groove-bottom sipe 16 can provide groove edges in the last
stage of tread wear life without excessively decreasing the rigidity of
the shoulder block 7a.

[0069] In order to secure the rigidity of the shoulder land region 7, the
groove-bottom sipe 16 in this embodiment is formed as a straight sipe
positioned on the center line G3 of the shoulder secondary groove 13. And
preferably, as shown in FIG. 3(a), both ends of the groove-bottom sipe 16
are closed ends which are respectively spaced apart from the
circumferential ends 7b and 7c of the shoulder secondary groove 13 by a
distance L5 of not less than 1.0 mm, more preferably not less than 2.0
mm, but not more than 8.0 mm, more preferably not more than 6.0 mm. If
the distance L5 becomes more than 8.0 mm, as the groove-bottom sipe 16
becomes short accordingly, the edge effect becomes insufficient. If the
distance L5 becomes less than 1.0 mm, the shoulder secondary groove 13 is
largely opened during cornering, and the inner part 11 and outer part 12
are decreased in the rigidity.

[0070] In order to effectively derive the above described advantages, as
shown in FIG. 3(b), the width w5 of the groove-bottom sipe 16 is
preferably set in a range of not less than 20%, more preferably not less
than 30%, but not more than 90%, more preferably not more than 80% of the
width w4 of the shoulder secondary groove 13.

[0071] Further, the depth D5 of the groove-bottom sipe 16 is preferably
set in a range of not less than 20%, more preferably not less than 40%,
but not more than 300%, more preferably not more than 250% of the depth
D4 of the shoulder secondary groove 13.

[0072] In order to increase the apparent stiffness of the shoulder blocks
7a, the shoulder axial groove 10 is preferably provided with a tie bar 17
rising from the bottom of the shoulder axial groove 10 and connecting the
adjacent shoulder blocks.

[0073] In this embodiment, since the inner part 11 is small, it is
preferable that the tie bar 17 is disposed near or adjacently to the
shoulder main groove 4 as shown in FIG. 2 and FIG. 3(a). The depth D6 of
the shoulder axial groove 10 at the tie bar 17 is preferably set in a
range of not less than 20%, more preferably not less than 30%, but not
more than 90%, more preferably not more than 80% of the depth D2 of the
shoulder main groove 4. The length L6 of the tie bar 17 is preferably set
in a range of not less than 30%, more preferably not less than 50%, but
not more than 120%, more preferably not more than 100% of the block width
ws2 of the inner part 11. If the axial length L6 of the tie bar 17 is
more than 120%, the shearing force of the snow packed into the shoulder
axial grooves 10 decreases.

[0074] Further, the inner part 11 may be provided with an inner sipe 27
extending from the shoulder main groove 4 toward the tread edge Te and
terminating without reaching to the shoulder secondary groove 13 so as to
divide the circumferential dimension of the inner part 11 into two
substantially equi-parts.

[0075] As shown in FIG. 4, it is preferable that the crown axial grooves 8
are crown sipes 18 whose width w7 is 0.3 to 1.6 mm. In this embodiment,
the crown sipes 18 are first crown sipes 18a and second crown sipes 18b
which are alternately arranged in the tire circumferential direction.

[0076] The first crown sipes 18a extend straight between the crown main
grooves 3 while inclining to one circumferential direction.

[0077] The second crown sipes 18b extend straight between the crown main
grooves 3 while inclining to the other circumferential direction.

[0078] By the first crown sipes 18a and second crown sipes 18b, crown
blocks 5a having an isosceles triangular shape are divided. Since the
crown axial grooves 8 have relatively narrow widths, it is possible to
minimize the decrease in the rigidity of the crown land region 5
subjected to a large ground pressure during straight running.
Accordingly, the steering stability on dry pavement can be maintained.

[0079] For that purpose, it is preferable that the width W7 of the crown
sipes 18 is decreased as far as possible. Therefore, the width w7 is more
preferably not less than 0.4 mm, but not more than 1.2 mm.

[0080] In order to derive the above mentioned effects, the depth of the
crown sipe 18 is preferably set in a range of not less than 20%, more
preferably not less than 30%, but not more than 90%, more preferably not
more than 80% of the depth of the crown main groove 3.

[0081] In order to effectively utilize the circumferential component of
the edges, the angle θ4 of the crown sipes 18 with respect to the
tire circumferential direction is preferably set in a range of not less
than 10 degrees, more preferably not less than 20 degrees, but not more
than 80 degrees, more preferably not more than 70 degrees.

[0082] The angle θ4 of the first crown sipes 18a is set to be the
same value as the angle θ4 of the second crown sipes 18b in order
to even the rigidity of the crown blocks 5a.

[0083] It is also preferable that the crown block 5a is provided with a
crown slot 19 extending straight from the crown main groove 3 toward the
tire equator c to further increase the edges and thereby to improve
on-the-snow performance.

[0084] If the crown slot 19 is too large, the rigidity of the crown block
5a decreases.

[0085] Therefore, the width w8 of the crown slot 19 is preferably not less
than 1.0 mm, more preferably not less than 1.5 mm, but not more than 5.0
mm, more preferably not more than 4.0 mm.

[0086] The depth of the crown slot 19 is preferably not less than 1.0 mm,
more preferably not less than 3.0 mm, but not more than 8.0 mm, more
preferably not more than 7.0 mm.

[0087] In order to prevent the rigidity of the crown block 5a from
decreasing, it is preferable that the crown slots 19 on all of the crown
blocks 5a are substantially parallel with the first or second crown
sipes.

[0088] The difference |θ5-θ4| between the angle θ5 of
the crown slots 19 and the above-mentioned angle θ4 is preferably
not more than 15 degrees, more preferably 0 degrees.

[0089] The crown block 5a may be provided with a crown slot 19 and a crown
cut 23. The crown cut 23 has a narrow width and extends between the crown
slot 19 and the crown axial groove 8 extending parallel with the crown
slot 19.

[0090] By the crown cuts 23, the axially outer edge 5a1 of the crown block
5a becomes uneven, and the snow grip performance can be improved although
the crown main grooves 3 are a straight groove.

[0091] During straight running, the ground pressure in the middle land
regions 6 becomes smaller than that in the crown land region 5,
therefore, it is permissible that the rigidity of the middle blocks 6a is
somewhat lower than the rigidity of the crown blocks 5a. It is therefore,
preferable that, as shown in FIG. 5, the middle axial grooves 9 include
first middle sipes 20 having a narrow width w9 as well as middle narrow
grooves 21 having a wider width w10. The width w9 of the first middle
sipes 20 is not less than 0.3 mm, preferably not less than 0.4 mm, but
not more than 1.6 mm, preferably not more than 1.0 mm. The depth of the
first middle sipes 20 is not less than 1.0 mm, preferably not less than
2.0 mm, but not more than 7.0 mm, preferably not more than 6.0 mm.

[0092] The width w10 of the middle narrow grooves 21 is more than 1.6 mm,
preferably not less than 1.8 mm, but not more than 5.0 mm, preferably not
more than 4.5 mm.

[0093] The depth D7 of the middle narrow grooves 21 is not less than 1.0
mm, preferably not less than 2.0 mm, but not more than 8.0 mm, preferably
not more than 7.0 mm.

[0094] As a result, by the wider middle narrow grooves 21, on-the-snow
performance such as snow grip can be improved, while retaining the
apparent rigidity of the middle blocks 6a as a whole owing to the narrow
first middle sipes 20.

[0095] In this embodiment, as shown in FIG. 5, the first middle sipes 20
and middle narrow grooves 21 are straight.

[0096] The angle θ6 of the first middle sipes 20 with respect to the
tire circumferential direction is preferably set in a range of not less
than 20 degrees, more preferably not less than 30 degrees, but not more
than 90 degrees, more preferably not more than 80 degrees.

[0097] The angle θ7 of the middle narrow grooves 21 with respect to
the tire circumferential direction is preferably set in a range of not
less than 10 degrees, more preferably not less than 20 degrees, but not
more than 90 degrees, more preferably not more than 80 degrees.

[0098] If the angle θ6 becomes less than 20 degrees, the lateral
stiffness (rigidity) of the middle blocks 6a becomes insufficient.

[0099] If the angle θ7 becomes less than 10 degrees, on-the-snow
performance such traction and breaking force is deteriorated.

[0100] It is preferable that the angles θ6 and θ7 are smaller
than the angle θ4 of the first and second crown sipes 18a and 18b.
Thereby, the middle blocks 6a subjected to larger ground pressure during
cornering, can be increased in the lateral stiffness (rigidity)
relatively to the crown blocks 5a, and the steering stability can be
improved.

[0101] Further, each of the middle narrow grooves 21 may be provided in a
central portion with a middle tie bar 21a rising from the groove bottom
to connect the adjacent blocks each other.

[0102] Further, it is preferable that each of the middle blocks 6a is
provided with a second middle sipe 22 extending straight from the crown
main groove 3 or shoulder main groove 4 and terminating within the middle
block 6a in order to further increase the edge component of the middle
block 6a and thereby further improve on-the-snow performance.

[0103] The axial length L8 of the second middle sipe 22 is preferably set
in a range of not less than 10%, more preferably not less than 20%, but
not more than 80%, more preferably not more than 70% of the axial width
wm of the middle block 6a. The depth of the second middle sipe 22 is
preferably set in a range of not less than 1.0 mm, more preferably not
less than 2.0 mm, but not more than 8.0 mm, more preferably not more than
7.0 mm. If the axial length L8 becomes more than 80%, it becomes
difficult to maintain the necessary rigidity of the middle block 6a.

[0104] In view of the rigidity of the middle block 6a, it is preferable
that the inclination of the angle θ8 of the second middle sipes 22
and the inclination of the angle θ7 of the middle narrow groove 21
are toward the same circumferential direction. Preferably, the difference
|θ7-θ8| between the angle θ7 and the angle θ8 is
not more than 30 degrees, more preferably not more than 20 degrees.

[0105] One of the axially inner edge 6a1 and axially outer edge 6a2 of the
middle block 6a which is longer than the other is partially provided with
an edge cut 25. In this embodiment, as shown in FIG. 5, the edge cut 25
extends from the opened end of the second middle sipe 22 to the opened
end of the middle narrow groove 21 so that the opened end of the second
middle sipe 22 shifts toward the inside of the middle block 6a and
thereby partial wear and chip-off starting therefrom can be effectively
prevented.

[0106] In this embodiment, further, the corner Z of the middle block 6a
between the middle narrow groove 21 and the crown main groove 3 is
provided with a chamfer 26 which has a triangular shape in the plan view
in order to prevent partial wear and chip-off starting from the corner.

Comparison Tests

[0107] Test tires of size 185/60R15 (rim size 15X6JJ) for passenger cars
having the tread pattern shown in FIG. 1 and the tread pattern shown in
FIG. 5 were prepared and tested as follows. The test tires had the same
specifications except for the specifications shown in Table 1.

[0134] In the test, a Japanese 1400cc FF car provided on the four wheels
with test tires (tire pressure 230 kPa) was run on a drying asphalt road
and snowy road in a tire test course. The steering stability was
evaluated by the test driver, based on the steering response, rigidity
and road grip during cornering.

[0135] Further, on the snowy road, the braking force and driving force
were evaluated.

[0136] The results are shown in Table 1 by an index based on the
comparative tire Ref. 1 being 100, wherein the larger value is better.